Propylene oxide (PO) is a versatile and useful commodity chemical. By volume, PO is among the top 50 chemicals produced in the world and is used primarily as an intermediate in the production of polyurethanes, propylene glycol, polyglycols, glycol ethers, allyl alcohols, and isopropanolamines. PO currently is manufactured by two main commercial processes: the chlorohydrin process and the hydroperoxide process.
In the chlorohydrin propylene oxide (CHPO) process, propylene and chlorine react in the presence of water to produce the two isomers of propylene chlorohydrin. This is followed by dehydrochlorination with lime or caustic to PO and the corresponding salt. A significant drawback of the chlorohydrin process is the large volume of water effluent (about 40 times the volume of PO produced), containing calcium chloride and/or sodium chloride along with other chlorinated organics. As a result, the technology is considered obsolete and only the hydroperoxide process is employed in newer PO production plants.
The hydroperoxide process, also referred to as indirect oxidation, proceeds via a two-step process: The first step requires the formation of a peroxide that is either hydrogen peroxide or an organic peroxide. In the second step the hydrogen peroxide is converted to water, or the organic peroxide is converted to the corresponding alcohol or acid, by epoxidation of propylene to PO. The two major routes responsible for the majority of current global production of PO are the propylene oxide/tert-butyl alcohol (PO/TBA) and the propylene oxide/styrene monomer (PO/SM) co-product processes. In both cases the co-products are formed in larger amounts than the PO itself, and therefore the economic viability of each route relies on co-product value and market demand.
Another technique for propylene oxide production is the cumene hydroperoxide route that is described as a co-product free hydroperoxidation route to PO and uses cumene hydroperoxide (CHP) as the oxidant. Co-product production is avoided by dehydrating and hydrogenating the cumyl alcohol, produced from the oxidation of propylene, back to cumene for reused.
Current industrial techniques for PO manufacture are deficient in a number of respects including: formation of undesirable by-products, propensity for PO to decompose and requirements for costly purification procedures. Both the chlorohydrin and the hydroperoxide processes are indirect processes. To date no direct oxidation process, i.e. reaction of propylene with oxygen, for PO manufacture has been commercialized. Severe technological limitations were shown to exist in prior attempts to create a direct oxidation process, including poor propylene conversion and selectivity.
Various peracetic acid epoxidation of propylene techniques are known. For example, U.S. Pat. No. 2,785,185 to Phillips et al, describes making oxirane compounds from olefins and aldehyde monoperacylates wherein PO is made by reacting propylene with an acetaldehyde monoperacetate in the presence of a catalyst in acetone solution in an autoclave reactor operating at 90° C. under pressure. The reactor material was then fractionally distilled to produce PO at a conversion of 43%, based on acetaldehyde monoperacetate. U.S. Pat. No. 3,341,556 to Stautzenberger and Richey describes a process for the production of PO by reaction of peracetic acid and propylene in the presence of a catalyst in an inert solvent. The reaction proceeds in a stainless steel autoclave reactor with reaction products separated by distillation. Finally, U.S. Pat. No. 3,663,574 to Yamagishi et al., describes the so-called The Daicel process for the continuous production of propylene oxide that involves bubbling propylene gas into an organic solvent solution containing 25% (by weight) peracetic acid and a stabilizer (tributyl phosphate) through a series of reaction columns. PO is separated from the liquid reaction mixture by absorption into butanol. In the Daicel process, no catalyst was used for epoxidation.